1. Field of the Invention
The present invention relates to a pressure casting process, especially relates to a drive control method for a squeeze pin used in the process.
2. Description of Related Art
Generally, in a pressure casting such as an operation in a die-casting machine, volume contraction arises in molten metal when it is injected into a mold cavity and solidifice therein. Then, shrinkage holes occur in mold article, which influence undesirable effects on strength and air-tight of the mold article.
Especially, in the die-casting machine, there is provided with a sharp slope of temperature, and shrinkage holes occur frequently. Therefore, various ways to avoid occurrence of such shrinkage holes have been proposed. A typical way is to use a pin which squeezes molten metal to homogenize matrix of a mold article by protruding into a cavity after the molten metal is injected and filled in the cavity. FIGS. 4 to FIG. 12 illustrate such way to use a squeeze pin.
FIG. 4 illustrates a main composition of a die-casting machine. In FIG. 4, numeral 1 designates a pair of mold dies. Molten metal 4 is teemed or injected into a cavity 2 formed by the mold dies 1. The mold dies 1 are formed with a moving mold die 5 and a fixed mold die 6. The moving mold die 5 is mounted on a moving die plate 51, and the fixed mold die 6 is mounted on a fixed die plate 61.
Operation for die clamping or die opening and closing is executed by moving the moving die plate 51. An injection sleeve 3 is mounted on the fixed die plate 61. An opening portion at the most right top of the sleeve 3 communicates through a gate 7 with the cavity 2. The molten metal 4 in the sleeve 3 is injected to the cavity 2 by an injection plunger 8 which is slidably inserted in the injection sleeve 8. The injection plunger 8 is driven by a injection cylinder 9 which is co-axially arranged with the injection sleeve 3. Numeral 10 designates a detector mounted on the injection cylinder 9 for detecting an injection pressure.
Also, in the mold dies 1, there are provided with a squeeze pin 11 and a squeeze cylinder 12 to drive the squeeze pin 11 between the moving mold die 5 and the moving die plate 51.
The squeeze pin 11 is inserted in a hole 13 located on the moving die 5 with its top capable of protruding into the cavity 2. The squeeze cylinder 12 and the squeeze pin 11 are arranged co-axially.
Also, there is provided with a stroke sensor 14 inside the squeeze cylinder 12 for detecting stroke quantities or length of the squeeze pin 11. This stroke sensor 14 is an absolute type of position detector using a differential transformer, which output signal to an identical stroke position is always kept to be the same because of determining the position of origin by the sensor itself.
Namely, as shown in FIG. 6, there are provided with a sleeve like coil portion 16 screwed into a bore formed inside a piston 17 of the squeeze cylinder 12, and a core 18 movably inserted therein, fixedly mounted on a cylinder head 15 of the squeeze cylinder 12.
Accordingly, the sleeve like coil portion 16 generates a signal corresponding to relative displacement between the core 18 and the coil portion 16 when the piston 17 moves in the cylinder 12.
The squeeze pin 11 is fixedly connected with the piston 17 (not shown in FIG. 6).
Therefore, the stroke quantity of squeeze pin 11 can be detected by means of a position of the piston 17. An increment type of stroke sensor may be used. However, such type of sensor needs a signal corresponding to the position of origin that brings about space or environment problems. Therefore, it is preferable to use the absolute type of stroke sensor.
In FIG. 4, each output signal of the stroke sensor 14 and the injection pressure sensor 10 is applied to an input unit 21 of a controller 20 which provides with a central processing unit (CPU) 22, a memory 23 and an output unit 24 besides the input unit 21. The output unit 24 is connected through an amplifier 25 to a solenoid valve 26 which directly controls the operation of the squeeze cylinder 12.
Also, numeral 27 designates a control apparatus for controlling various operations of a die-casting machine, which is connected to the controller 20 to input various control information such as initial value S0 of optimal stroke length and etc.
FIGS. 8 (a) and (b) show wave forms of the injection pressure P and the piston stroke S0 in relation to a drive control operation of the squeeze pin 11. As shown FIG. 8, an count timer 201 starts its time counting operation at the time when the injection pressure P reaches a set value P0 (at the time of switching to boosting). When the time counting operation reaches time up state, that is, a time interval T0 passes, the count timer 201 generates a signal so as to switch the solenoid valve 26, which in turn causes the squeeze cylinder 12 to move the squeeze pin 11, thereby squeeze operation being executed.
FIG. 7 shows a control block diagram of the controller 20. In FIG. 7, actual stroke quantity Sa of the squeeze pin 11 is detected by the stroke detector 14 and then it is fed back to confirm whether or not the detected value Sa is within a target zone (allowance zone) S0xc2x1xcex1. Based on the confirmation, a correct means 200 supplies to the count timer 201 a corrected time interval T0xc2x1xcex94T where xcex94T is small time unit as a parameter stored in the memory 23. Then, next step of injection molding is executed.
In the next step, the count timer 201 starts its time counting operation at the time when the injection pressure P reaches the set value P0 stored in the memory 23. When the corrected time interval T0+xcex94T elapses by the counting operation, the count timer 201 generates the signal so as to switch the solenoid valve 26, which in turn causes the squeeze cylinder 12 to move the squeeze pin 11, thereby squeeze operation being executed in accordance with the corrected time interval.
In a trial molding process, each molding step is repeated by modifying the time interval in such a way described above so that the stroke of actual squeeze pin 11 comes into the optimal target zone S0+xcex1.
FIG. 5 is a flow chart showing a process for drive control of the squeeze pin 11. At the step 1 of FIG. 5, parameters such as injection pressure P0, optimal stroke S0, allowance zone xc2x1xcex1 for the optimal stroke S0, code N0 of mold die 1, time interval T0 corresponding to time up of count timer 201, small time unit xcex94 T for modifying the time interval T0 and shot repeat number n for modifying the time interval T0 are preset. At the step 2, injection operation starts. Then, at the step 3, the injection pressure P detected by the pressure senor 10 is compared with the preset value P0. When detected injection pressure P reaches preset value P0, the count timer 201 starts to the counting operation. Then, at the step 4 it is judged that the count timer 201 becomes time up or not. In case of time up, at the step 5, the timer 201 supplies a signal through the output unit 24 to the solenoid valve 26 which causes the squeeze cylinder 12 to move the squeeze pin 11. As the squeeze pin 11 moves, molten metal 4 in the cavity 2 is squeezed.
The squeezed molten metal 4 is also on the way to solidification. When the solidification progresses to an extent, the squeeze pin 11 can not move further and stops. At the step 6, the stroke sensor 14 detects a stroke Sa of the squeeze pin 11 when it stops. Then, at the step 7, the detected stroke Sa is compared with the optimal stroke S0 stored in the memory 23 at the CPU 22, and judged whether or not the detected stroke Sa is within the target zone S0xc2x1xcex1. In case of the stroke Sa to be within the target zone, the initial preset value of time interval T0xc2x1xcex94T is stored in the memory 23 at the step 10 and then, next injection molding process is started.
In case of the stroke Sa to be out of the target zone, at the step 9 a new time interval is defined by adding or reducing by a small time unit xcex94T to the time interval T0, and then, the next injection molding process is started.
Namely, in case that the stroke Sa is less than the lower target point S0xe2x88x92xcex1, it may be happened that the solidification completes before the squeeze pin 11 reaches preset stroke S0. Accordingly, the new time interval is defined as T0xe2x88x92xcex94T so as to advance starting time of the squeeze pin 11.
On the other hand, in case that the stroke Sa is larger than the upper target point S0+xcex1, it may be happened that the solidification doe not complete when the squeeze pin 11 reaches preset stroke S0. Accordingly, the new time interval is defined as T0+xcex94T so as to delay starting time of the squeeze pin 11.
As described above, reference set stroke is defied as a sum of the preset stroke S0 and its allowance xc2x1xcex1. The controller 20 always executes feed back control so that detected actual stroke Sa of the squeeze pin 11 comes into the target zone S0xc2x1xcex1.
Further, there is provided with a display unit 200 capable of displaying the detected stroke Sa in a digital ways, thereby making easy to compare and evaluate with real products
In the above description of the flow chart shown in FIG. 5, the number of shot xe2x80x9cnxe2x80x9d at the step 8 is assumed to be xe2x80x9c1xe2x80x9d, and in case of the judgment at the step 7 to be xe2x80x9cNOxe2x80x9d, modification of the time interval at the step 9 is executed at each shot. However, in case of the number xe2x80x9cnxe2x80x9d to be plural, for instance, xe2x80x9c8xe2x80x9d, the time interval T0 is modified at every 3 shots based on two consecutive shots ahead.
Furthermore, a specific criterion may be used. Then, under the criterion, detected actual time intervals are stored in the memory 23 in relation to each mold die and molding conditions thereof; which are used as initial values in the next injection molding.
In the above description, the present P of injection cylinder 9 is used as a criterion to determine the time interval of the squeeze pin 11. However, other physical elements such as hydraulic power, injection speed, injection stroke and distortion of mold die or pin for pushing out an article could be used as the criterion. Also, in the above description, the stroke sensor 14 is arranged in the squeeze cylinder 12. However, such arrangement of the stroke sensor 14 has a drawback of mounting a precise detector inside the mold die. Japanese patent laid open No.8-132211, discloses a squeeze pin drive method which is unnecessary to use the stroke sensor 14. In this method, as shown in FIG. 4, there is provided with a flow meter counter 100 for measuring flow rate of pressurized hydraulic fluid supplied to squeeze cylinder 12. The stroke quantity of squeeze pin 11 can be measured indirectly by means of the flow meter counter 100. Furthermore, No.8-182211 discloses a method in which time interval to start the squeeze pin, pressure P of hydraulic fluid supplied to squeeze cylinder or its flow rate Q could be changed when difference between the detected stroke of squeeze pin and the set stroke thereof is over a defined stroke quantity.
FIG. 9 illustrates a control block diagram for controlling the squeeze cylinder using the above physical quantities T, P, Q and particularly, the dashed line portion Z indicates a control portion concerning compensation quantities xcex94 T, xcex94P, xcex94Q. FIG. 10 is a time chart for explaining stroke operation of the squeeze pin with respect to FIG. 9. FIG. 11 illustrates a stroke waveform S in accordance with one stroke operation of squeeze pin closingly following to solidification and shrink or contraction of molten metal in a mold cavity. In more detail, the stroke operation starts after injection pressure P reaches preset value P1 and time interval elapses. Then, the stroke operation progresses until a desired stroke end within a target zone.
In a stroke control operation of the squeeze pin above described, if a difference between actual detected stroke and its target value exists, in other words, if the stroke position is out of target zone, the correction or modification required is executed so as to gradually approach a desired position in such a manner that, as shown in FIG. 9, a unit quantity xcex94A T, xcex94P, or xcex94Q for compensating, each being predetermined as a parameter, is added or reduced unit by unit to each corresponding preset value.
Namely, in the stroke control operation, even if full of the deviation to target quantity measured during one shot operation is applied for the next stroke operation, the actual stroke in the next short does not necessarily converge to the target, because the solidification process of molten metal filled in a cavity is too complex to correctly understand both of the temperature lowering and the volume contraction, and under the present level of metal and die-casting technology it is inevitable to use an indirect controlled variable such as T, P, or Q.
Accordingly, as shown in FIG. 9, it is practical to asymptotically approach a target stroke by applying a small unit quantity of compensation factor xcex94T, xcex94P, or xcex94Q for one time correction.
However, in trial molding, such an asymptotical approach requires many times of shot operation for reaching target stroke after initial setting, as shown in FIG. 12. (FIG. 12 illustrates an asymptotical approach more than 10 times to reach target zone.)
An object of the present invention is to provide a new drive control method of a squeeze pin which overcomes the drawback that many times of shot operation for reaching target stroke are required during trial molding by means of an asymptotical approach mentioned above.
To achieve the above object, a drive control method of a squeeze pin according to the present invention is constituted as followings.
In a drive control method for pushing a squeeze pin slidably mounted on a squeeze cylinder in a mold die into a mold cavity formed in conformity with a desired mold article in accordance with solidification processes of molten metal in the cavity after the molten metal is filled therein, the method comprising the steps of:
setting a target point or a target zone on a stroke quantity of the squeeze pin for molding the mold article,
defining values of a controlled variable in accordance with at least one of time lag T, flow rate Q and supply pressure P those are concerned in driving the squeeze pin, wherein the time lag T is a time interval between a time instance when injection pressure reaches a predetermined value and a time instance when the squeeze pin starts to operate, the flow rate Q is a flow rate of pressurized hydraulic fluid supplied to the squeeze cylinder and the supply pressure P is pressure values of the hydraulic fluid supplied to the squeeze cylinder,
executing first trial molding for the article one or several times based on the defined values of the controlled variable, and detecting each actual stroke quantity of the squeeze pin during the trial molding,
firstly modifying or holding the controlled variable by defining intermediate values between the defined values and maximum values of the controlled variable to be allowed as new values of the controlled variable in case that actual stroke quantities detected during the first trial molding are less than the target point or zone, by defining intermediate values between the defined values and minimum values of the controlled variable to be allowed as new values of the controlled variable in case that the actual stroke quantities are larger than the target point or zone and by holding the defined values in case that the actual stroke quantities are within the target point or zone,
executing second trial molding for the article one or several times based on the firstly modified or hold values of the controlled variable, and detecting each actual stroke quantity of the squeeze pin during the second trial molding,
secondly modifying the controlled variable by defining intermediate values between the modified values and the values immediately before the firstly modifying as new values of the controlled variable in case that the actual stroke quantities of the squeeze pin during the second trial molding are out of target point or zone,
executing third trial molding for the article one or several times based on the secondly modified values of the controlled variable, and as the result holding the secondly modified values as that of controlled variable thereafter in case that the actual stroke quantities of the squeeze pin during the third trial molding are within the target point or zone, and figuring out intermediate values between the secondly modified values of controlled variable and the values immediately before the secondly modifying in case that the actual stroke quantities of the squeeze pin during the third trial molding are out of target point or zone.
It is preferable that the firstly modifying comprises figuring out difference between values of controlled variable and values of target point or zone, and modifying values of controlled variable so as to be proportional to the difference.
According to the present invention, the numbers of shot operation necessary for reaching target stroke in trial molding are greatly reduced.